Re: Re[2]: how many sensitive cells?

[Lester.Slaback@nist.gov]

>I believe the original question asked why BOTH to total mass and total
>energy weren't considered.  I suggest that when the exposure is NOT
>uniform either through attenuation or localization and then the EDE
>calculations are done, they effectively average the dose (concentration)
>over the whole body mass.  This is how one gets micro R of dose from
>100's of mR of exposure in dental x-ray for instance.
>THIS process mathmatically DOES consider the total energy and total mass
>and when used by LNT believers is equated to risk.  Same for population
>dose.
>Since the basic dose unit is a concentration unit and the risk estimates
>are based on this concentration unit - calculation of total energy is
>meaningless.
>Ted de Castro

Re:the original sin:
The question was (in a left handed way) why is not risk proportional to
the total number of cells subject to irradiation times the actual fluence
at the cells.

Assuming one does the corrections for partial exposures, depth
distribution, and all the other things that affect the number of cells
that might be exposed at a particular fluence, why is this not the
starting point for a risk estimate.

As pointed out doctors, when they administer a dose, are administering a
total energy (i.e., a pill) and make corrections for body wieght, etc. 
Whereas HPs treat dose as energy per unit mass.  I suggest that for the
quality of our risk knowledge that these are equally valid approaches. But
the underlying presumption is that risk is in fact proportional to the
number of cells irradiated. Again, let us assume that all the appropriate
fluence corrections are made so we are looking at the product of [cells
exposed]*[effective fluence]. Obviously this is just a starting point
since non-linear corrections for rate, repair processes, etc still must be
made.

But if one agrees that this proportionality is a starting point then one
must accept all the approximations that get us to our current system, and
must accept that the current system is just that - an approximation.

I think Ted's comment above in fact is agreeing with this underlying
presumption. He is pointing out that the external field is not the same as
that at the cells.

But I saw no comments on what I was trying to find out - Is the total
number of cells  in the body that are possible proginators for cancer a
constant across adults of widely varying sizes?  Or a constant to a
reasonable approximation?

Related to the risk question is the regulatory/NCRP/ICRP limits.  If risk
is related to the total number of cells exposed why are limits based on
dose to a point even for partial body exposures?  Does anyone believe that
1 cm2 of skin exposure has the same risk as a whole body skin exposure at
the same dose?
I can accept this as a regulatory convenience as long as that is
explicitly acknowledged. But that is not the way it is typically viewed.

(Just stirring the pot a bit more.)
-- 
the above are the personal musing of the author,
and do not represent any past, current, or future
position of NIST, the U.S. Government, or anyone else
who might think that they are in a position of authority.
NBSR Health Physics
NIST
Gaithersburg, MD 20899
301 975-5810
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Lester.Slaback@nist.gov
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